5 μg/l was what the EPA originally proposed an arsenic standard but in January 2001, it was changed to 10 μg/l. The consensus of EPA, was to get the MCL down to 5 μg/L. Enter a new President and a new director of the EPA, and in 2010, IRIS issues an "External Review Draft" called the "Toxicological Review of Inorganic Arsenic (Cancer). It is important to not here, that this draft has been in the making since 1999. A different administration views the risk differently. Which is one of my points. How should we quantify risk for water and rice that is reasonable, prudent, and...well...reflects reality.

These cancer risks are based on a Slope Factor, which is "the theoretical cancer potency estimate for humans." (CalEPA)

I'm going to argue that this "theoretical cancer potency estimate" paints us into a corner when attempting to communicate the risk of exposure, especially the risk to a chemical suspected of causing cancer in humans.

It is not that it is "theoretical" that I take issue with, it is that the number it derives - the excess cancer risk - is not a threshold whereby exceeding it by 1, 5, or 10 ppb becomes "troubling," "worrisome," "cause for concern," or "potentially harmful" as Consumer Reports points out that it does.

If I am going to support my conclusion that the rice is "safe," I am going to have to show why exceeding the 5 ppb threshold is not harmful. The thing is, I can't do that. I have been painted into a corner that says there is no safe exposure to a carcinogen. This corner is also where New Jersey finds itself when it says the drinking water concentration that results in a one-in-one-million excess lifetime risk meets their law as an acceptable risk.

This corner gives us no where to go based on this thinking. This is why Dr. Honnycutt with the Texas Commission on Environmental Quality made this statement to the EPA about their 2010 External Review Draft on the "Toxicological Review of Inorganic Arsenic (Cancer)."

Like I said previously in my past posts, exceeding 0.003 μg/L increases the risk. Accepting the threshold of 5 μg/L as the "most protective level" assumes that one then accepts a new risk threshold of 2 excess cancers per 1,000 (2E-03) based on consuming two (2) liters of water at 5 μg/L. This is based on a Slope Factor that I calculated based on a 1 in 1,000,000 risk at 0,003 μg/L (I assumed 2 liter/day).

Let's instead look at the risk using the new and improved draft IRIS values, which are based on an Oral Cancer Slope Factor for women:

Based on the IRIS data, which is based on data from Morales et al. (2000), drinking on liter of water, or consuming one serving of rice (Consumer Reports) with 0.14 μg/L (ppb) would present an excess cancer risk for lung and bladder in women of 1 in 10,000 (10-4, the acceptable risk for carcinogens in water.)

Using that same data, consuming one serving of rice that had the highest concentration of inorganic arsenic (9.6 μg) for a lifetime - 70 years- would see an excess cancer risk of 7.3 in 1,000 in women.

Looking at it another way, if the "unit risk" for arsenic is 7.3 in 10,000 for one μg of arsenic in water, 10 μg would increase that risk 10-fold - 73 in 10,000 or 7.3 in 1,000.

If we can accept that unit risk, which was derived from data provided by Morales et al. (2000), then we can figure out what the excess cancer risk is at New Jersey's "most protective level" of 5 μg/L:
5 μg/L x 7.3E-04 = 36.5E-04 or 3.6 in 1,000.

Using the 5-ppb standard in our study, we found that a single serving of some rices could give an average adult almost one and a half times the inorganic arsenic he or she would get from a whole day’s consumption of water, about 1 liter.

Therefore, one serving of rice that contained the maximum concentration of inorganic arsenic detected, 9.6 μg:

...would present a risk of 7 in 1,000 excess bladder/lung deaths for females - which is 3 excess bladder/lung cancers than what the "most protective level" of 5 μg would theoretically produce. This is all based on the following assumptions:

The female weighs 70 kg.

The female eats one serving of rice containing 9.6 μg inorganic arsenic - every day - for 70 years.

The female gets bladder or lung cancer.

The female dies from bladder or lung cancer.

All of those assumptions must be met to get an excess cancer risk of 7 in 1,000. If she was to consume one serving of rice were with 5 μg of inorganic arsenic - every day - for 70 years, we would see an excess cancer risk of 3.6 in 1,000 which Consumer Reports tells us is at the "most protective level."

And, even if all of those assumptions play out, that excess cancer risk is dependent on a Slope Factor that was developed from data produced by Morales et al.

About Me

I've been dealing in one form or fashion with hazardous waste since 1984. I have recently completed my Masters in Public Health (MSPH) from the Texas A&M HSC-School of Rural Public Health. I'm a CHMM and a CET and currently spend my time teaching and developing environmental related courses.